1. Field of the Invention
The present invention relates to a method of softening or loosening catalyst pellets caked and adhered to a catalyst floor or support for facilitating the safe removal of the spent catalyst in a desulfurization system for a residual oil and more particularly it relates to a method of softening or loosening a caked catalyst by washing the caked catalyst using a washing oil containing an organic amine and/or ammonia gas absorbed therein.
In addition, the term "the softening or loosening of caked catalyst" as used herein means the softening of spent catalyst pellets and the loosening or unbinding of these catalyst pellets caked by caking materials as described hereinafter and for the sake of simplicity the term "the softening of caked catalyst" will be used herein.
2. Description of the Prior Art
In general, the removal of a spent catalyst from a desulfurization reaction system has been previously carried out by a method in which the spent catalyst is washed out by circulating a gas oil under suitable temperature and pressure conditions after removing the heavy oil feed from the reaction system or a method in which after washing the spent catalyst with a gas oil and opening the reactor, the caked catalyst in the reactor is mechanically crushed and removed. However, in the method in which a gas oil is circulated through the catalyst, the effect of the method varies depending upon the shape of the catalyst pellets, that is, the method may be effective for the treatment of the catalyst having, for example, a comparatively large particle size but the method is not so effective for softening or loosening the cakes of a catalyst having a small particle size. On the other hand, the mechanical crushing method reduces the quality of the working environment since the crushing operation is usually done manually and a fine dust is formed. Thus, this method is undesirable from the standpoint of safety and health.
As the results of investigations on the reasons causing the catalyst to cake, it has now been found that the caking of catalyst pellets occurs when the spaces among the catalyst pellets are closely filled with carbon, sulfur components, etc., contained in the heavy oil feed during the desulfurization reaction. In particular, it was found that carbon adheres tightly and rigidly to the catalyst pellets as a result of the crystallization of the carbon and also the sulfur components form compounds with iron, vanadium, etc., contained in the heavy oil feed or become dispersed in the carbon in the form of free sulfur, which results in the formation of concrete-like caked catalyst pellets. Thus, attempts were made to remove the caking materials filling the spaces between the catalyst pellets of the caked catalyst by washing the caked catalyst with an aqueous alkali metal hydroxide solution and as the result thereof it has been found additionally that the surface of the caked catalyst is water repellent since the surface is covered by carbon, etc., Hence, the catalyst pellet cake is not loosened or the caking material filling the spaces between the pellets is not dissolved out if the caked catalyst is not contacted with the alkali metal hydroxide solution for a long period of time. Moreover, since an alkali metal hydroxide solution partially dissolves the amorphous alumina which is the carrier of the catalyst, the treatment of the spent catalyst and the washings used causes troublesome problems.